This invention relates to a process and apparatus for the destruction of solid waste material by burning the combustibles in a gravity-type, multiple hearth furnace.
Heretofore in the art to which my invention relates, the conventional operation of such furnaces includes three distinct process functions, namely, drying and preheating the feed materials; burning of the volatiles in the feed materials; and cooling the inert ash. Prior to being burned or oxidized in the burning section, the solid matter flows through a pre-heating zone where it is preheated and dried by the flow of hot waste gas from the burning zone or by other means. Such a process is particularly adapted for the burning of substances that are high in inerts and low in calorific value, such as filter cake derived from sewage sludge filtration, with such solids being burned to nearly 100%. The process preferably takes place in a multiple hearth furnace which is simple to operate and has high thermal efficiency. The waste gas rising from the combustion zone usually passes directly through the preheating zone where it moves in countercurrent flow to the solid fuel which is introduced from the top of the furnace. A disadvantage in such a system lies in the fact that the preheating and drying process takes place at temperatures between 300.degree. and 1100.degree. F which results in the release of gaseous and vaporous substances having odors which rise and mingle with the waste gas and create an environmental nuisance, unless such noxious substances are prevented from contaminating the atmosphere by means of an after-burner or similar system. The addition of such equipment for removal of noxious substances brings about increases in both the initial cost and operating cost of the system and also a decline in the operating efficiency of the system. Heretofore, the cooling of the inert ash in a conventional multiple hearth furnace has been accomplished in the lower hearts where waste heat in the ash is removed by preheating air which is introduced into the furnace through the lowermost hearths thereof.